51% attack and local Blockchain

[turvarya]

I recently was thinking about the 51% attack and about full node.
Everybody, who e.g. uses Bitcoin Core has a full copy of the Blockchain on his PC. When I got the 51% attack right, it means that somebody has that much hashing power, so he could overrule the actual blockchain by making a longer chain(proof of work means, that the longest chain is true).
I was wondering:
What happens to all the local blockchains, when that happens? Do the blocks, that differ from the winning blockchain get deleted and replaced with the new ones? Can Bitcoin Core delete already downloaded blocks?

[TonyT]

I recently was thinking about the 51% attack and about full node.
Everybody, who e.g. uses Bitcoin Core has a full copy of the Blockchain on his PC. When I got the 51% attack right, it means that somebody has that much hashing power, so he could overrule the actual blockchain by making a longer chain(proof of work means, that the longest chain is true).
I was wondering:
What happens to all the local blockchains, when that happens? Do the blocks, that differ from the winning blockchain get deleted and replaced with the new ones? Can Bitcoin Core delete already downloaded blocks?

Good question, and I suspect that yes, the way the program is written, every time you connect to the internet the Bitcoin Core will automatically 'synch up' with the most recent blockchain, and if that blockchain is compromised, the old, 'real' blockchain will be deleted and/or modified.

BTW, not to change the thread too much, but what's wrong with a 51% attack?  Unless the attacker was Al Qaeda, or North Korea, it would make more sense for the attacker to not rock the boat too much, and just slowly steal coins once in a while, but not too much, and post a notice saying:  "I will randomly steal 1% of any bitcoin transaction from time to time, but I will not steal 100%, so please, bitcoin user, continue using bitcoin."  And then, for PR effect, the 51% attacker can say: "I am taking 1% because I intend to give the money to a worthy charity and/or improve bitcoin, blah blah blah".   Right?

[amaclin]

What happens to all the local blockchains, when that happens?

Reorganizing blockchain is trivial process. Every day there are several "conflicting chains"
https://blockchain.info/orphaned-blocks
Right now they happen because of network latency, not because of 51% attack

Do the blocks, that differ from the winning blockchain get deleted and replaced with the new ones? Can Bitcoin Core delete already downloaded blocks?

Up to client. Current Bitcoin Core Client keeps these blocks in blk-files. Because there is a possibility to reorganize them back into the mainchain

Can Bitcoin Core delete already downloaded blocks?

Yes it "can", but there is no reason for it. So, it does not delete them.

[turvarya]

I recently was thinking about the 51% attack and about full node.
Everybody, who e.g. uses Bitcoin Core has a full copy of the Blockchain on his PC. When I got the 51% attack right, it means that somebody has that much hashing power, so he could overrule the actual blockchain by making a longer chain(proof of work means, that the longest chain is true).
I was wondering:
What happens to all the local blockchains, when that happens? Do the blocks, that differ from the winning blockchain get deleted and replaced with the new ones? Can Bitcoin Core delete already downloaded blocks?

Good question, and I suspect that yes, the way the program is written, every time you connect to the internet the Bitcoin Core will automatically 'synch up' with the most recent blockchain, and if that blockchain is compromised, the old, 'real' blockchain will be deleted and/or modified.

BTW, not to change the thread too much, but what's wrong with a 51% attack?  Unless the attacker was Al Qaeda, or North Korea, it would make more sense for the attacker to not rock the boat too much, and just slowly steal coins once in a while, but not too much, and post a notice saying:  "I will randomly steal 1% of any bitcoin transaction from time to time, but I will not steal 100%, so please, bitcoin user, continue using bitcoin."  And then, for PR effect, the 51% attacker can say: "I am taking 1% because I intend to give the money to a worthy charity and/or improve bitcoin, blah blah blah".   Right?

You can not steal bitcoin with the 51% attack, you can just double spend(making two transactions with the "same" bitcoins, so, one of the supposed recivers don't get it.

But you reminded me of another question:
Can't you instead of double spending bitcoins, just reverse(/delete) a transaction?

Do the blocks, that differ from the winning blockchain get deleted and replaced with the new ones? Can Bitcoin Core delete already downloaded blocks?

Up to client. Current Bitcoin Core Client keeps these blocks in blk-files. Because there is a possibility to reorganize them back into the mainchain

So, I have a lot of unused blk-files, when I run Bitcoin Core for a while, or do these blk-files get cleaned up, after a while?

[amaclin]

So, I have a lot of unused blk-files, when I run Bitcoin Core for a while, or do these blk-files get cleaned up, after a while?

To be correct: some parts of your blk-files are not used.
Current implementation does not defragment your local blk-files.
You can do it manually, but it is not reasonable.

[amaclin]

You can not steal bitcoin with the 51% attack, you can just double spend(making two transactions with the "same" bitcoins, so, one of the supposed recivers don't get it.

1. You can "steal" coinbase-emission bitcoins (25btc rewards) with 51% attack. OK, the word "steal" is not correct in bitcoin system.
2. With 51%-attack you gain profits not by doubling your bitcoins, but by the things you receive twice for them.

[susan234]

I recently was thinking about the 51% attack and about full node.
Everybody, who e.g. uses Bitcoin Core has a full copy of the Blockchain on his PC. When I got the 51% attack right, it means that somebody has that much hashing power, so he could overrule the actual blockchain by making a longer chain(proof of work means, that the longest chain is true).
I was wondering:
What happens to all the local blockchains, when that happens? Do the blocks, that differ from the winning blockchain get deleted and replaced with the new ones? Can Bitcoin Core delete already downloaded blocks?

i think the blocks  that differ from the winning blockchain won't been deleted ,and the BTC on your wallet you saw,maybe
disappear, and you  don't know if you don't re-download blocks

[Argwai96]

You can not steal bitcoin with the 51% attack, you can just double spend(making two transactions with the "same" bitcoins, so, one of the supposed recivers don't get it.

1. You can "steal" coinbase-emission bitcoins (25btc rewards) with 51% attack. OK, the word "steal" is not correct in bitcoin system.
2. With 51%-attack you gain profits not by doubling your bitcoins, but by the things you receive twice for them.

In order for a potential 51% attack to be most efficient you will first need to own and spend bitcoin but even then it is not 100% sure that you can execute a double spend transaction as the various inputs of transactions are often intertwined and it would be possible that you would essentially not receive the value from your original spend when you try to double spend if the entity you are attacking does a good job in managing their inputs

[zinger]

What happen when someone attacks blockchain? Can the attacker gets something from it?

[turvarya]

What happen when someone attacks blockchain? Can the attacker gets something from it?

No, you can't steal bitcoins by attacking the blockchain. You can attack transactions, delay them, even remove them, but you just can't make a transaction without having the private key.